1. Field of Invention
The present invention relates to optical units capable of markedly enhancing utilization efficiency of rays of light and improving luminance, and liquid crystal display modules using the same.
2. Description of the Related Art
Liquid crystal display (LCD) modules have been frequently used as a flat panel display through utilizing features such as thinness, lightness in weight, low level of electric power consumption and the like, and applications thereof have been increasing year by year as display devices for information such as mobile phones, personal digital assistances (PDA), personal computers and televisions. In recent years, properties demanded for liquid crystal display modules vary depending on their applications, but may include brightness (high luminance), visibility (widening of viewing angle), energy saving, thin and light modeling capability and the like. In particular, high luminance has been strongly desired.
Conventional common liquid crystal display modules have a structure in which as shown FIG. 8, a liquid crystal display element 51, a variety of optical sheets 52 and a backlight 53 are superposed from the front face side to the back face in this order. The liquid crystal display element 51 has a structure in which liquid crystal cell 56 is sandwiched between a pair of polarizing plates 54, 55, and various display modes such as TN and IPS were proposed. The backlight 53 allows the liquid crystal display element 51 to execute light emission by irradiating from the back face side, and modes such as edge light type (side light type), immediate beneath type and the like have been in widespread use. The various optical sheets 52 are superposed between the liquid crystal display element 51 and the backlight 53, and a light diffusion sheet, a prism sheet and the like are included having an optical function such as refraction and diffusion in an orientation along a normal line for the purpose of allowing the rays of light exited from the front face of the backlight 53 to enter into the entire face of the liquid crystal display element 51 efficiently and uniformly.
Generally used polarizing plates 54, 55 provided in the liquid crystal display element 51 are those which exhibit absorption dichroism, i.e., absorption of one directional component of a light accompanied by transmission of remaining polarization components. This type of the polarizing plates 54, 55 absorb 50% of the light, in principle, for achieving polarization, therefore, it falls under one great reason for deterioration of the utilization efficiency of the light in the liquid crystal display module.
In order to minimize the decrease in utilization efficiency of the light by the polarizing plates 54, 55, a technique of superposing a reflection polarizing plate (polarization splitter) on the back face side of the polarizing plate 55 positioned on the back face side in the liquid crystal display module, as well as a technique of using the reflection polarizing plate in place of the polarizing plate 55 positioned on the back face side were developed (for example, see JP-A-2005-106959, JP-T 9-506985 (the term “JP-T” as used herein means a published Japanese translation of a PCT application) and the like). According to this reflection polarizing plate, the polarizing plate 55 positioned on the back face side allows transmission axis components to transmit directly, while other polarization components are allowed to return to the lower side, thereby recycling the rays of light.
On the other hand, the optical sheets 52 such as a light diffusion sheet, a prism sheet and the like provided in the liquid crystal display module generally include a transparent substrate film made of a synthetic resin, optical layer(s) such as a light diffusion layer, a prism array layer and the like laminated on the front face of this substrate layer (see, for example, JP-A Nos. 2000-89007, 2004-4970 and the like). The conventional optical sheet 52 is constructed such that the optical layer having a particular structure performs optical functions such as refraction, diffusion and the like in an orientation along a normal line, however, control of polarization characteristics of transmitted rays of light was not intended thereby.
In conventional liquid crystal display modules, the luminance was not satisfactorily secured while maintaining requirements for LCD, i.e., thin and light modeling by merely improving optical waveguide plate, cold-cathode tube and the like of the backlight 53.
Also, under the current circumstances, even in the case of the aforementioned liquid crystal display modules in which such reflection polarizing plates are used, just approximately 75% of the utilization efficiency of the rays of light can be achieved in fact due to loss of the recycled light caused by thermal absorption, reflection and the like.